From my experience, the only way is to make all trees invulnerable per trigger. D&D will damage everything, no matter how you change the allowed targets.
If you don't want that, I guess you'll have to trigger the spell.
You could use the default spell and trigger the damage only? That'll make the triggering much easier to do....but I'm not very familiar with the mechanics of death and decay, so I'm not sure how easy it would be to get the targets hit by it.
I'm on a page about incorrect corrections, and spent the better part of like two hours trying to get someone to understand that -5^2 = -25, not 25, and then that post had comments get reposted because that group is self sustaining, and that person was in turn trying to explain what I just explained to them. And I'm taking that as a victory
I will be AFK for a couple of days you guys hold down the fort while I am gone. I will be checking in on my phone but that will severly limit me. Be back saturday!